Causes of & Reasons for Small Breasts


Breasts come in all shapes and sizes. Some are round, some are pointed, some are asymmetrical. Some are large and some are small. There is absolutely nothing wrong with your breasts or nipples based on their size or shape.

Some of us can self-perceive that we have underdeveloped breasts if we note a discrepancy between our own body image, and the internalized images of appropriate or desirable breast size and shape. It’s worth noting that these images can come from less than reliable sources, embedding an unrealistic standard of “beauty” in our minds.

How is Breast Size Measured?

Before we can really talk about breast size, we should first mention how it’s measured. Breast size is usually stated in terms of size of bra.

For a deeper dive on measuring your bra size, click here.

Breast size usually focuses on the cup size of your breasts. However, it is worth nothing that this size can fluctuate over a woman’s lifetime and especially during pregnancy.

What Are Underdeveloped Breasts Anyway?

“Small” is a pretty relative term and it may not mean the same thing to different people. If I’m a F or G cup, I might consider C cups small. If you’re a C, you might think an A cup is small. 

The average bra size in the U.S. is 34DD. However, of course there are many women significantly smaller, and larger than this size (it is after all a mere average).

Not to mention that the same cup size might look smaller and bigger depending on your frame and the shape of your breasts. So “small” needs to be better explained and qualified.

One misconception about small breasts that it’s worth clearing up: You’re not more or less capable of nursing with large breasts or small breasts. Breast size doesn’t indicate much breast milk you can produce and large breasts aren't automatically better for the job.

What Determines Breast Size?

Okay, now you know your breast size, you might wonder WHY? Why is one person a 36A and another a 32E. In a way, this is like wondering why we don’t all have the same size and shape noses… because we’re all different! 

So what causes such extreme differences in bust size? What causes small breasts? The answer is really in your genes. Small breasts or even absent breast development can sometimes be caused by underlying genetic conditions.


Yep, you can mostly blame (or thank) your genes for both the size and shape of your breasts, and even the composition of your breast tissue. 

According to Nature, “Breast size is a highly heritable trait. A twin study previously estimated the heritability of bra cup size to be 56%8. Several genome-wide association studies have also identified common genetic variants associated with breast size.”


Another major factor that can be both nature and nurture is body weight. Breasts are complex body parts but they are composed significantly of adipose fat. This may mean that individuals with higher body fat will often have larger breasts.

Are Small Breasts a Medical Concern?

Micromastia (also called hypomastia, breast aplasia, breast hypoplasia, or mammary hypoplasia) is a medical term describing the underdeveloped breasts. However, there is no formal definition of micromastia in terms of size.

Some congenital causes include ulnar–mammary syndrome (caused by mutations in the TBX3 gene), Poland syndrome, Turner syndrome, and congenital adrenal hyperplasia

Hormonal Imbalance

If an hormonal imbalance is responsible for inhibiting growth of breasts, it will manifest with other symptoms such as hirsutism (excessive hair growth on a woman’s body and face).

If this is the case your doctor may refer you to an endocrinologist (who specialises in hormonal problems), or, maybe a gynaecological endocrinologist (who specialises in hormonal disorders of females).

Poland Syndrome

Poland syndrome is a disorder in which affected individuals are born with missing or underdeveloped muscles on one side of the body. According to the NIH, “People with Poland syndrome are typically missing part of one of the major chest muscles, called the pectoralis major.”

If this affects the breasts it will be one sided, resulting in asymmetry. Sometimes, milder cases of Poland Syndrome become more evident during puberty. 

What Can Cause Breast Size to Change?

While breast size is mostly determined by your genes, that doesn’t mean it stays the same throughout your life. Indeed, most women experience many changes. Here are some of the key causes of changes to your breast size (and shape).

Age: Puberty to Menopause

From puberty through maturity and into menopause, our bodies go through many changes. These developmental stages are often ushered in by hormonal changes and hormonal changes can definitely affect your boobs. 

Menstrual Cycle

Hormones also play a major role in your menstrual cycle and these can cause monthly changes to your breast size. Many women especially notice breast swelling and tenderness just before their period as part of their premenstrual symptoms (PMS) and/or during ovulation, around the middle of their cycle.

Hormonal Birth Control

Hormonal birth control is used to prevent unwanted pregnancy but the hormones in play may also cause fluid retention and breast swelling in some women. These effects are often stronger in the first few months of taking a new birth control pill.


During pregnancy, your breasts may double or triple in size. According to the Mayo Clinic,

“One of the first signs of pregnancy is an increase in breast size. As early as two weeks after conception, your breasts start to grow and change in preparation for producing milk. Stimulated by the hormones estrogen and progesterone, the milk-producing glands inside your breasts get bigger and fatty tissue increases slightly.

By the end of your first trimester, your breasts and nipples will be noticeably larger, and they may keep growing throughout your pregnancy. Breast enlargement accounts for at least a pound of the weight you gain while pregnant.”

Your breasts may remain enlarged for a while after birth.

Weight Gain or Loss

Weight fluctuation is one of the many reasons it’s recommended you go for a bra fitting regularly (every year or so). This is because the fat component of a woman’s breasts may make it a body area she’ll easily lose or gain weight.

However, it’s worth noting that it is difficult-to-impossible to target fat gain or loss to a specific body area. So if you lose or gain weight it will be distributed throughout your body, affecting other areas as well as breasts.


Among the many health risks associated with smoking is a loss in skin elasticity, which can affect the delicate skin and breast tissue. This may cause breasts to sag. Some women perceive this as a change in size, though it’s really more related to shape unless accompanied by other changes.

Breast Augmentation

Of course, one of the more obvious ways to change breast size is to go under the knife. Whether it’s a breast augmentation or reduction, this will cause a more extreme and often permanent change to your breast size. If you are considering a breast augmentation, please seek out medical advice.